The present invention relates generally to the multidisciplinary field of quantum computing which includes the fields of quantum physics and computer science. More specifically, the present invention relates to quantum computing for performing quantum computations.
Quantum computing, the use of quantum physical systems to represent and process information, has been a focus of research for approximately the last twenty years. An impetus for recent research has been the realization that computational problems exist that can be solved with qualitatively greater efficiency than by conventional digital computers. These computational problems include, for example, the Grover search algorithm and the Shor factorization algorithm.
A practical method for carrying out quantum computation, however, has proven elusive due to the quantum nature of the systems required. See, e.g., xe2x80x9cThe Physical Implementation of Quantum Computation,xe2x80x9d DiVincenzo, D. P., quant-ph/0002077, Feb. 25, 2000. Decoherence of quantum states can occur due to interactions of physical systems with the surrounding environment. This caused difficulty in carrying out quantum computation in practice. For example, decoherence of quantum states has prevented trapped ion devices from successfully implementing quantum computation. See, e.g., U.S. Pat. No. 5,793,091, entitled xe2x80x9cParallel Architecture for Quantum Computers using Ion Trap Arraysxe2x80x9d which issued on Aug. 11, 1998.
Another impediment in achieving practical quantum computation relates to the difficulty in preparing the required initial quantum state for performing quantum computation. For example, this has prevented nuclear magnetic resonance (NMR) devices from successfully implementing quantum computation experimentally. See, e.g., U.S. Pat. No. 5,917,322, entitled xe2x80x9cMethod and Applications for Quantum Information Processingxe2x80x9d which issued on Jun. 29, 1999.
One known alternative approach has been the use of quantum states in an optical device to implement quantum computation. By using weak coherent states, the necessary initial quantum state can be approximated and subsequently processed with negligible quantum decoherence. Consequently, previous impediments due to decoherence can be overcome.
These known optical-based systems, however, suffer from several shortcomings. For example, these known optical-based systems are semiclassical systems (consequently referred to as quantum computation xe2x80x9csimulatorsxe2x80x9d) that fail to accomplish many of the expected benefits of an actual quantum computational system. See, e.g., xe2x80x9cOptical Simulation of Quantum Logicxe2x80x9d, Cerf, N. J., et al., Physical Review A, Vol. 57, March 1998, PACS numbers: 03.65.Bz, 42.50.-p., 42.79.Ta,89.70+c; xe2x80x9cQuantum Computation with Linear Opticsxe2x80x9d, Adami, C. and Cerf N. J., quant-ph/9806048, Jun. 14, 1998; both of which are incorporated herein by reference for background purposes.
Moreover, these known optical-based systems fail to scale effectively; in other words, the physical size of these known systems increase more quickly than linearly with respect to the number of quantum bits (i.e., xe2x80x9cqubitsxe2x80x9d) of information to be processed. Said another way, as quantum computers having an ever greater number of qubits are considered, the number of system components physically required increases non-linearly, in some cases even exponentially. Consequently, constructing an actual quantum computer having a practical number of qubits has been physically prohibitive using known systems due to the physically unacceptable size and number of system components required.
At least one qubit in a quantum computing device is created. At least one photon is placed into a superposition of quantum states. The quantum states each have an associated probability amplitude. The quantum states each are associated with a mode from a group of orthogonal modes. The probability amplitudes associated with the quantum states of the at least one photon are temporally separated thereby forming at least one qubit, the alternative values of which are thus temporally identifiable.